The gallbladder of Necturus, a typical leaky sodium transporting epithelium, will be employed in vitro as a simple model to study the passive and active electrical properties of the cell membranes and the paracellular shunt pathway. Our final goal is to be able to describe quantitatively these membranes, the transepithelial ion transport process and its control mechanisms. Conventional microelectrode techniques will be used to characterize further the passive permeation of ions across each membrane, i.e., to establish their partial ionic conductances. In addition, the effects of simple experimental perturbations on the properties of the different electrical pathways of the tissue will be studied, in an attempt to learn about the control mechanisms of these properties. Intracellular ion activities will be measured with ion-selective microelectrodes, in order to establish (from the activities and the measured membrane potentials) which ions are at electrochemical equilibrium. Deviations of the measured distribution from the predicted passive distribution under steady state conditions will allow us to establish that the species is actively transported. Intracellular micro-iontophoresis will be employed to selectively increase the intracellular activity of single ions, and to measure the effects on the potential and the ion activity gradients across the membrane. These experiments can allow us to demonstrate rheogenic components in the active ion transport mechanisms. Analogies in the transport and electrical properties of gallbladders and other different species of vertebrates, and similarities between the gallbladder and other epithelia allow us to think that our results will be applicable, in general, to leaky sodium-transporting epithelia as a whole.